1. Field of the Invention
The present invention relates to a semiconductor device and its manufacture, and more particularly to a semiconductor device with a multi-layer wiring structure and its manufacture method.
2. Description of the Related Art
Improvements on multi-layer conductive wires are being made along with higher integration of semiconductor integrated circuit devices. Self-aligned contact (SAC) techniques are also adopted to form a contact in a narrow area. There are strong requests for reduction of the manufacturing cost of semiconductor integrated circuit devices. In order to lower the manufacturing cost, it is effective to reduce the number of masks.
FIGS. 23A and 23B show an example of the structure of a dynamic random access memory (DRAM) according to conventional techniques. FIG. 23A is a vertical cross sectional view taken along a direction parallel to a bit line, and FIG. 23B is a vertical cross sectional view taken along a direction perpendicular to the bit line.
A bit line 111a shown in FIG. 23A and an insulated gate electrode (word line) 104 shown in FIG. 23B do not actually appear in these cross sectional views, but are hidden in the background regions. However, these bit line 111a and gate electrode 104 are drawn for purposes of easy understanding.
As shown in FIG. 23A, a shallow trench isolation (STI) region 102 of an oxide film is buried in the surface layer of a p-type region 101 of a semiconductor substrate. The STI region 102 defines or surrounds respective active regions. On the surface of the active region, a gate oxide film and a gate electrode layer are laminated and patterned to form a pair of gate electrode structures each constituted of a gate oxide film 104a and a gate electrode 104b. An etching stopper film of silicon nitride or the like may be formed covering the upper and side surfaces of the insulated gate electrode. By using the gate electrode 104b as a mask, n-type impurity ions are implanted into the exposed active region. As a result, a source/drain region 103a to be connected to the bit line is formed in the central area of the active region, and a pair of source/drain regions 103b to be connected to storage capacitors are formed on both sides of the gate electrodes. A first interlayer insulating film 105 of silicon oxide or the like is formed on the substrate surface, covering the gate electrode.
A resist pattern or the like is formed on the first interlayer insulating film 105, and contact holes to the source/drain regions 103a and 103b are formed through the first interlayer insulating film 105. Thereafter, conductive material such as polysilicon is deposited, and the conductive material on the first interlayer insulating film 105 is removed to leave lower plugs 107 and 108 only in the contact holes.
Thereafter, a second interlayer insulating film 109a of silicon oxide or the like is deposited on the first interlayer insulating film. A mask pattern is formed on the silicon oxide film 109a. A trench is formed through the silicon oxide film 109a in order to form, for example, a bit line. After this mask pattern is removed, another mask pattern is formed having an opening corresponding to a contact area in the bottom area of the trench. By using this other mask pattern, the second interlayer insulating film 109a is etched to form a bit line contact hole. A bit line contact 111b is formed in the bit line contact hole, and a bit line 111a is formed in the bit line trench. These contact 111b and bit line 111a may be made of the same material or different materials.
A third interlayer insulating film 109b is deposited covering the bit line 111a. If SAC techniques are to be incorporated, the upper and side surfaces of the bit line 111a are covered with an etch stopper film.
A resist layer is formed on the surface of the third interlayer insulating film 109b, and an opening is formed through the interlayer insulating films 109a and 109b in an area corresponding to the storage electrode lower plug 108. This opening is buried with a storage capacitor contact 114 to form a surface electrically connected to the source/drain region 103b in the surface of the third interlayer insulating film 109b. 
Thereafter, an insulating film 113 of silicon oxide or the like is formed, and a portion of the insulating film 113 where a storage electrode is to be formed, is removed. A storage electrode 116 is deposited, and the storage electrode on the upper surface of the insulting film 113 is removed. The surface of the storage electrode is covered with a capacitor dielectric film 117, and a plate electrode 118 is formed on the capacitor dielectric film 117.
FIG. 23B shows the cross sectional structure perpendicular to that shown in FIG. 23A. The storage electrode contact 114 is formed between a pair of bit lines 111a. As shown in FIG. 23A, by limiting the area occupied by the storage electrode contact 114, a parasitic capacitance between the storage electrode and bit line can be suppressed small.
However, the structure shown in FIGS. 23A and 23B requires two masks, one for forming the storage electrode 116 and the other for forming the storage electrode contact 114.
FIGS. 24A and 24B show the structure of DRAM whose storage electrode and its contact are formed by one mask.
FIG. 24A is a cross sectional view taken along a direction parallel to a bit line 111a, and FIG. 24B is a cross sectional view taken along a direction perpendicular to the bit line 111a. As shown in FIGS. 24A and 24B, in this structure, the same cross sectional shape of a storage electrode 116 extends downward to form a storage electrode contact 114. Therefore, the same mask can be used for forming the storage electrode and storage electrode contact, reducing the number of masks by one.
However, as shown in FIG. 24A, the storage electrode contact 114 extends in parallel to the bit lines 111a in a large area and a parasitic capacitance therebetween increases. If an insulating film 109c formed in contact with the side walls of the bit line 111a is made of material having a large dielectric constant, such as silicon nitride, the parasitic capacitance increases greatly. If the insulating film 109c is made of silicon oxide film, there is no etch selectivity so that electrical shortage is likely to be formed between the storage electrode contact and bit line.
A hole continuous in a depth direction and having different cross sectional shapes generally requires two masks. If such a hole is to be formed by using one mask, it is difficult to control the cross sectional shape of the hole. If the number of masks is increased, the manufacturing cost is difficult to be lowered, whereas if the number of masks is reduced, desired electric characteristics are difficult to be obtained.